Add false as return value for mb_convert_encoding

[kesselb]

mb_convert_encoding could return false: https://www.php.net/manual/en/function.mb-convert-encoding

https://psalm.dev/r/1887ce60a3

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/1887ce60a3

<?php

$x = mb_convert_encoding('test123', 'UTF7-IMAP', 'UTF-8');
if ($x === false) {
 echo 'Nooooo';   
}

Psalm output (using commit 0e8ca1d):

ERROR: TypeDoesNotContainType - 4:5 - string does not contain false

[muglug]

Thanks!